1. Field of the Invention
This invention relates to a method of making chemical-vapor-deposited (CVD) nitride (e.g. titanium nitride) films having highly stable properties including film resistance and thickness, and more particularly to the use of a high-power, long-duration plasma treatment step to reduce the tendency of the resistance and thickness of the as-deposited nitride film to change with time.
2. Description of the Prior Art
Chemical vapor deposition (CVD) is a material synthesis method in which the constituents of the vapor phase react to form a solid film at the surface of a substrate. The growth of thin films by CVD has been one of the most important methods of film formation in the fabrication of integrated circuits (ICs) such as very large system integration (VLSI) and ultra-large system integration (ULSI) devices. The importance of the CVD technology is ever increasing as the IC line width is shrinking toward quarter-micron and beyond.
The rapidly growing importance of CVD lies primarily in its versatility for depositing a large variety of elements and compounds at relative low temperatures to form both vitreous and crystalline layers having a high degree of perfection and purity. Compared to other deposition techniques, e.g. sputtering deposition, it is generally easier to use CVD to create films having better crystallinities and more accurately controlled stoichiometric compositions. As a result, films made by CVD generally have better-controlled physical and material properties. Thus, while sputtering deposition is used primarily for the deposition of a limited number of metal films, e.g., aluminum-copper-silicon (Al--Cu--Si) and titanium (Ti) films, in VLSI or ULSI devices, a much greater percentage of dielectric and conductive films in such devices are now formed by CVD, including silicon oxide (SiO.sub.2), silicon nitride (Si.sub.3 N.sub.4), phosphosilicate glass (PSG), titanium nitride (TiN), borophosphosilicate glass (BPSG), and tungsten (W) films.
Moreover, as the IC line width shrinks below 0.2 .mu.m, TiN films deposited by physical vapor deposition (PVD) techniques upon via holes or contact holes can no longer provide a topographical coverage that satisfies process requirements imposed by the subsequent tungsten plug process. As a consequence, CVD processes are gradually replacing PVD as the standard method for making TiN thin films in IC devices.
A typical prior-art CVD TiN film formation process involves the thermal decomposition of one or more titanium-nitride-based organometallic compounds, e.g., tetrakis-dimethylamino titanium (TDMAT), followed by the formation of a carbon-containing TiN film. The as-deposited, porous TiN film is then treated by nitrogen and hydrogen plasma to reduce its organic carbon content. Typically, the nitrogen gas flow rate is 200-300 sccm, the hydrogen gas flow rate is 300-400 sccm, the plasma power density is approximately 200 W or under, and the plasma treatment time is approximately 30 sec or under. However, the TiN film formed after such a plasma treatment tends to absorb oxygen in the atmosphere if vacuum is broken. Such intake of oxygen invariably causes surface resistance and resistivity of the film to rise, resulting in an increase in the RC value of the IC device. Significantly, breaking of vacuum is virtually inevitable in IC processing. This is because a great number of different process steps must be performed to make a modem IC device and because vacuum is generally inevitably broken when the wafer is transferred from one process equipment to another. Thus, breaking of vacuum has a significant adverse impact on TiN thin films deposited by a conventional CVD process.
FIG. 1 illustrates the adverse effect of exposure to atmosphere on several TiN films formed by a conventional CVD process. As indicated by line 2 in FIG. 1, the initial RC value of the TiN film deposited upon a 0.65 .mu.m via hole is approximately 30 ohms. After a breaking of vacuum for 24 hours, the RC value of the film is almost doubled at 60 ohms. Similar changes are observed in TiN films deposited on a 0.6 .mu.m (line 4) and a 0.65 .mu.m (line 6) via hole, respectively.
In addition to the aforesaid resistance change problem in TiN films caused by breaking of vacuum, the resistance and resistivity of the TiN film may also increase because of other process-related reasons. For example, a rapid thermal nitridation (RTN) process is typically conducted to make a barrier layer on top of an as-deposited TiN film. Such RTN process may also raise the surface resistance of the as-deposited TiN film.